Does anyone know which APTL exams I should focus on first (I want to do the hardest ones first if possible). I might get this help from the course anyway.
That's rather subjective as everyone has at least one subject that they struggle with to varying degrees. One pilot might argue Air Law is hard while another will say they can't understand Met while someone with a physics background will tell you principles of flight is the easiest. I suggest you think back to your PPL and identify what you found difficult and use that as a jumping off point. Yes some exams are harder than others, but you have to know your strengths and weaknesses before you take on any advice for what order to write exams in.
Good luck.